Compress an MP4 file

Why your MP4 is bigger than you expected

MP4 is a container, not a codec — a wrapper that holds a video stream, an audio stream and the timing that keeps them in step. Nothing about the wrapper determines file size. What determines it is the bitrate the video stream was encoded at, and MP4 files arrive at high bitrates for four common reasons.

• Export defaults. Editing software and screen recorders ship with generous presets because a too-large file is a recoverable mistake and a too-ugly one is not. A default 1080p export at 20 Mbps is roughly four times what the footage needs.
• Resolution you are not using. 4K is four times the pixels of 1080p. If the video is going into a 700-pixel-wide column, three quarters of the file is describing detail that will never reach a screen.
• Frame rate. 60 fps costs meaningfully more than 30 fps for content that does not move — a screen recording of someone scrolling a document gains nothing from the extra frames.
• Length. The one nobody can argue with. File size is bitrate times duration, and duration is not a setting.

How much smaller an MP4 gets

Honest answer: it depends on the source, and anyone quoting you a single percentage is guessing. What is predictable is the shape of it. If your MP4 came straight out of a camera, a phone or a screen recorder, there is usually a great deal of slack — halving the bitrate is often invisible. If it came out of a compressor already, there is not, and you are trading picture quality for megabytes at close to a one-to-one rate.

The way to find out is to run a short test rather than reason about it. Compress the file once at the default setting and look at the reported saving; the tool shows you the before and after size and the percentage between them before you download anything.

Re-encoding an MP4 that is already H.264

Most MP4 files already contain H.264, and compressing here re-encodes them to H.264 again by default. That is a generational loss: the encoder is working from an image that already carries the previous encoder's artefacts, so it faithfully preserves those artefacts and then adds its own on top. Switching the codec to H.265 does not escape it, because the damage is already in the picture being handed over.

This is not a reason to avoid compressing — one extra generation at a sensible bitrate is barely visible, and it is the price of the file being smaller. It is a reason to compress once, from the highest-quality source you still have, rather than repeatedly nudging the same file down in steps. If you have the original export, start from that rather than from the copy that has already been through a compressor.

When remux does nothing, and when it does everything

Remux copies the video stream untouched and rewrites only the container. For MP4 to MP4 that is close to a no-op — you get the same bytes in a freshly written wrapper, at the same size.

Where it earns its place is going the other way: an MKV or MOV that already contains H.264 and only needs to become an MP4 so a browser or a phone will play it. That takes seconds, loses nothing, and does not reduce the file size at all — which is the whole point.

Settings for an MP4

Leave the output format on MP4 unless you have a reason to change it — H.264 in MP4 is the one combination you can hand to any device made in the last fifteen years and expect it to play. The codec control offers H.265 next to it, which is worth taking only when the file is for your own archive: it gets you roughly a third off at the same quality, and it costs you several times the encode time plus every device that cannot decode it.

If you have a size limit, switch to target-size mode and type it. If you just want the file smaller, use Best compression and start with the quality slider in the middle. Drop the resolution one step whenever the video will be watched in a window rather than full-screen; it is the single most effective control on this page. If the clip is longer than the part anyone needs, set the trim points before you encode, because the bitrate budget is solved from the trimmed length.

MP4 compression questions

Compressing to a size limit instead?

If what brought you here is a number rather than the fact that it is a MP4 file, start from the ceiling instead. Each of these opens the compressor with that target already set, ready for your MP4:

• 5 MB
• 8 MB
• 10 MB
• 16 MB
• 20 MB
• 25 MB
• 50 MB
• 100 MB
• 200 MB
• 500 MB

Other formats

• Compress MOV iPhone and QuickTime recordings. Frequently HEVC or ProRes, and frequently huge.
• Compress MKV Long recordings, rips and archives. Big enough that upload-based tools often refuse them.
• Compress WebM Already efficient — so the realistic gains are smaller, and worth understanding first.
• Compress AVI Legacy codecs, dramatic savings. Read in, MP4 out.
• Compress WMV Old Windows exports. Nothing plays them now, and remuxing is not an option.
• Compress FLV A dead container, often with a perfectly modern video inside. Try remux first.
• Compress M4V Apple's MP4 under another name. Renaming it often solves the whole problem.
• Compress MTS / M2TS AVCHD camcorder footage. Enormous, usually interlaced, and split into 2 GB pieces.

The formats reference puts MP4 beside the other containers and shows which codec combinations play where.